According to both the Lexus USA press release and a post on Toyota’s Open Road Blog, there will be an IS 350C (at least in North America):

Powered by the same powertrain as the IS sedans, the IS 250C features a2.5-liter, 204-hp V6 mated to a six-speed manual or six-speed automatictransmission; the IS 350C contains a 3.5-liter, 306-hp with a six-speedautomatic.

Hardly surprising, but at least this clears up any confusion. (The reason for the IS 350C’s omission from the Paris Auto Show is due to that engine configuration not being offered in Europe.)